There are from 80,000 to 150,000 total hip arthroplasty (THA) procedures completed each year in the United States. Worldwide, there are over 300,000 to 400,000 THA procedures performed each year. This procedure, whether unilateral or bilateral, is performed to relive a variety of objective signs of disability, such as to relieve pain and to increase or preserve mobility. A diagnosis which suggests THA for treatment may include, for instance, primary or secondary osteoarthritis, congential dysplasia, polyarthritis, including rheumatoid arthritis, and ankylosing spondylitis, previous unsuccessful joint surgery and Paget's Disease of the bone. The indications for bilateral THA include primary idiopathic bilateral monoarticular osteoarthritis, primary generalized osteoarthritis, ischemic necrossis of the femoral head with secondary acetabular failure, and secondary degenerative osteo arthritis resulting from congential dysplasia. Additional factors such as flexion deformity of more than 30.degree., one hip fixed in addjuction, the other fixed in abduction (causing the patient to fall from lack of balance), leg shortening, acetabular protrusion, age and other factors should be considered prior to considering a bilateral procedure.
Generally stated, THA is an operation where the ball and socket joint which forms the natural hip is replaced by artificial materials. The development of THA components has occurred over the last three decades. The THA procedure and related components are described in the publication entitled The Howmedica Precision Hip System, Copyrighted 1986 by Howmedica, Inc., which publication is hereby incorporated by reference in its entirety. In the procedure carried out today, the spherical end of the femur is removed and replaced with an artificial metallic implant. The stem to which the ball is attached (via a "neck") fits down the middle of the femur in the surgically prepared medullary canal and is located in place by bone cement. The spherical head of the femoral component is placed into the socket of the acetabular component forming a total replacement of the hip joint (both components are commercially available as the A.T.S..RTM. Total Hip System and The P.C.A..RTM. Total Hip System from Howmedica, Inc.).
In preparing the femoral medullary canal for implantation, the steps after resection (removal) of the head of the femur include reaming, broaching and cleaning out (lavage and brush) the medullary canal. This area of the bone contains the bone marrow which also fills spaces in cancellous bone.
The femoral canal typically is prepared from its distal portion ot he proximal portion, i.e., in a retrograde manner. The femoral canal may be opened with any standard blunt surgical awl or manual reamer. The surgeon, in cleaning the medullary canal, will progressively use larger reamers until the reamer contacts the harder bone at the cortex of the isthmus. Broaches or rasps are utilized in order to accommodate the appropriate implant with neutral, posterior and anterior implants. At the proximal end (with reference to the surgeon) of the medullary canal, the proximal broach is used. This broach has a smooth tip and middle portion with its cutting surface being proximal. This instrument has five functions: it provides the precise canal size for the cement mantle; it is used to position the calcar reamer; the flat plane can be used to provide the final osteotomy level and it is used in trial reduction to assure proper fit and as a trail since it is sized to correspond to the correct femoral component pluys cement mantle. The prepared (reamed and broached) medullary canal is then cleaned and dried.
Prior to introducing cement into the intramedullary plug is typically utilized to effectively create a block at the isthumus (a lower portion fo the medullary canal at which it has a narrow diameter) to make the upper femur a closed system. The plug reduces the amount of debris forced up the canal when cement is introduced and it insures pressurization thus helping to provide stem fixation.
The actual bone cemen tis not a glue but is a filler and enables the mechanical interlock of bone on one side to a prosthesis on the other. The material used in creating the bone/prosthesis interface which is presently preferred is polymethylmethacrylate (also known as PMMA), one of a family of the polymers known as acrylics and is familiar commercially as Plexiglass.RTM. and Lucite.RTM.. This material is "cold-curing" or "self-polymerizing" thus enabling its use in the THA procedure. A preferred cement utilized today is Surgical Simplex.RTM. P Bone Cement (commercially available from Howmedica, Inc.), which is a co-polymer of polymethylmethacrylate and styrene. This amterial has a compressive strength ranging from 9,000 to about 13,800 pounds per square inch, a tensile strength ranging from about 3,600 to 6,800 psi; a shear strength ranging from 5,700 to 7,000 psi and a modulus orf elasticity ranging from 2.3 to 3.8.times.10.sup.5 psi.
A variety fo factors and variables will influence the effectiveness fo the cement used in the THA Proceuree, such as: the rate of mixing; the porosity of the cement formed during mixing; the additives utilized in conjunction with the bone cement such as the addition of antibiotics to the cement mix; polymer shrinkage; the THA procedures itself; the set time; the powder to liquid ratio; tprepartion of the bone surface including the presentation of debris such as blood, bone chips or powder and other tissue; the delay in aopplying the cement; the pressure at which the cement is applied into the medullary canal and the cement thickness. The pressure at which the cement is supplied to the medullary canal is one of the most significant factors in the success of the implant.
In the early days of prosthetic surgery, the mixed cement was placed into the femur and simply manually pressed into place. No matter how much pressure is manually applied sufficient pressure to insure a good interface between cement and bone could not be assured. Furthermore, use of this technique in the past often resulted in a femoral canal that was incompletely filled with cement.
The cement is now typically provided to the interior exposed endosteal surface of the medullary canal with a bone cement gun (commercially available, for example, as the Exeter.RTM. Cement Gun & Syringe from Howmedica, Inc.). In the commercially available cement guns, a nozzle is fitted to the gun and delivers the cement under pressure to the canal. The liquid and powder which compriess the cement may be mixed prior to placement in the cement gun with commercially available systems (i.e., the Mix-Kit.RTM. Systems or in the Simplex Enhancement Mixer.RTM. commercially available from Howmedica, Inc.) so that it may be applied in a viscous or liquid state.
After the bone cement is applied to the exposed endosteal surface of the prepared medullary canal, the implant is inserted into the canal. The cement, which polymerizes and hardens in the space between the bone and the implant, functions as a luting agent. The quality of the fixation is greatly enhanced by the mechanical interlocking of the cement with tthe porous trabecular structure of the cancellous bone of the wall of the intramedullary canal and with any pores, dimples, elevations, keys, etc. provided ont eh surface of the implant.
Fixation of surgical implants with polymethylmethacrylate bone cements within intramedullary canals has been practiced with great success for many years. On occasion, however, problems associated with the premature lossening of the implant in use have been observed. One explanation for these loosening problems is an inadequate penetration fo the bone cement into the cancellous bone of the intramedullary canal wall. It is known that this penetration can be improved by pressurizing the viscous or liquid bone cement within the intramedullary canal so as to work the cement deeply into the cancellous bone fo the canal wall before it hardens. Thus, it is well known to utilize an intramedullary plug as described above to prevent passage of cement distally (with reference to the surgeon) of its desired location within the intramedullary canal (see, for example, U.S. Pat. Nos. 4,245,359; 4,276,659 and 4,293,962 and European Pat. No. 6408).
Pressurization can be further improved to some but, as noted above, a limited extent by finger packing by the surgeon. Compactors have also been used to compress and pressurize bone cement applied to an intramedullary canal. However, the use of a compactor requires the addition of a distinct, time-consumign step to the surgical procedure, with the results being operator intensive, i.e., the extennt of pressurization achieved depends upon the axial force exerted by the surgeon.
Additionally, it is known to equip the nozzle of a bone cement extruder with a restrictor (e.g., the Miller Bone cement Injector Restrictor Set; Zimmer, USA; Warsaw, Ind.) made of a solid resiliebnt material to block the flow of cement between the nozzle and the bone through the open end of the prepared intramedulary canal. However the quality fo the seal obtained is limited because the fits of such a frestricter againt the bprepared bone is more in the nature of a line contact at the open end than a surface-to-surface contact and, furthermore, the quality of the seal will be reduced when the restricter is unable to completely fill any irregularities in the bone against which it fits. Again, the extent of pressurization achieved depends upon the axial force exerted by the surgeon.
Devices simlar to the Miller device have been utilized wherein the upper or proximal portion of the sal is more flexible than on the Miller seal in order to accommodate a wider variety of openings in the medullary canal. This type of seal remains within the medullary canal by virtue of axial pressure fromt eh surgeon holding it in place or alternatively by its fit wtihin the prepared medullary canal. The pressure of the cement added to the bone would oppose, however, the fit of this type of plug pushing it in a direction out of the proximal end of the medullary canal.
In addition, U.S. Pat. No. 4,462,394 discloses an intramedullary canal seal which comprises a hollow tube adapted to slidingly receive the nozzle of a bone cement extruder and an inflatable cuff surrouidng the tube and a means to inflate the cuff. The inflated cuff is said to form a seal against the wall of the intramedullary canal, threby preventing escape of cement through the open end of the prepared canal. In European Patent Application No. 82304353.4, a device is disclosed which is designed to fit over and seal the opening of a cavity in a bone to allow pressurization of cement in the cavity. This device is described as having an aperture for sealingly receiving a cement delivery nozzle and the seal member itself may be a balloon seal, which is inflatable and expandable, or a solid material, either of which embodiments is urged or pressed against the opening of the bone by force of the barrel of the cement delivery gun or an additional abutment means. This device does not fit within the intramedullary canal but instead seats on top of the proximal end of the canal. the cement sealing effect is achieved by the force of holding this eal against the opening of the bone and not by any force exhibited form the cement against the seal.
Cement restrictors have also been disclosed for use in conjunction with fixing the acetabular portion of the hip prosthesis as in U.S. Pat. Nos. 3,889,665 and 3,886,248. These restrictors do not provide for the use of the cement pressure itself to hold the seal in place during application of the bone cement.
I have invented a sealing device which avoids the aforementioned problems and provides for much greater penetration of cement into the bone.